This invention relates to a process for preparing fuel materials, and more particularly, to the preparation of fuel materials from sweet sorghum bagasse.
As fossil fuel sources become increasingly scarce, it will be necessary to produce alternative energy materials. Alternative materials must be inexpensive, readily available, and preferably derived from renewable sources. They must also be sufficiently non-polluting.
Wood and wood by-products are prime examples of renewable energy materials presently in use. L. H. Reineke, in "Wood Fuel Preparation", U.S. Forest Service Research Note FPL-090, Forest Products Laboratory, Forest Service--U.S. Dept. of Agriculture, pp. 1-14, describes the preparation of wood fuels. Reineke indicates that wood is usable in various forms including chips, sawdust, shavings, and briquets. Reineke further describes the preparation of wood briquets in "Briquets From Wood Residue", U.S. Forest Service Research Note FPL-075, November 1964, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture. Another reference describing wood and fibre-based fuel materials is U.S. Pat. No. 3,227,530 to B. H. Levelton, entitled "Process of Producing Fuel Logs". This patent discloses a process in which plant materials are shaped and formed into fuel logs.
However, there are distinct disadvantages in using wood-based fuel products. As wood fuel consumption increases, raw material costs will increase proportionately. In addition, the availability of particulate wood by-products is dependent on the increasing use of such materials in particleboard and fibreboard. Finally, trees require a long growth period. This long period decreases the desirability of wood as a renewable energy soure.
As an alternative to wood-based products, other agricultural materials have been examined. Both sugarcane and sweet sorghum are agricultural products which have been studied. According to R. W. Schery, Plants for Man, Prentice-Hall, Inc., Second Ed., 1972, pp. 371-374, sugarcane is a coarse grass grown in humid tropical lowlands. Schery indicates that 1 to 11/2 years are required to grow and harvest sugarcane. However, sugarcane requires temperate, humid growing conditions. It is not adaptable to growth in colder regions of the United States.
In comparison with sugarcane, sweet sorghum is more resistant to colder temperatures. It also requires less humid climates. According to R. L. Monk, et al., "Improvements of Sorghum for Energy Production", Texas Agricultural Experiment Station--Texas A&M University, sorghum (Sorghum bicolor (L.) Moench) is a tropical grass which may be grouped into three basic types. These types include grain, forage, and sweet sorghum. However, over 22,000 varieties of sorghum exist throughout the world. D. R. Jackson, et al., in "Development of Sweet Sorghum as an Energy Crop, Volume 1: Agricultural Task", Research Report for U.S. Department of Energy, Battelle Laboratories Columbus Division, May 31, 1980, page 22, describes sweet sorghum as a member of the grass family which is drought tolerant and adaptable to most major agricultural regions of the United States.
Historically, sorghum is indigenous to Africa. S. Kresovich, et al., in "Sweet Sorghum Breeding Line Evaluations: 1981-1982", Texas Agricultural Experiment Station--Texas A&M University, PR-4181, December 1983, indicates that the culture of sweet sorghum was first introduced to the United States in 1853. Early uses of sweet sorghum involved syrup production, according to J. P. Clark, et al., "Construction and Demonstration of a Modern Community-Sized Sorghum Sirup Plant", Virginia Polytechnic Institute and State University. However, syrup production later diminished for practical and economic reasons. Clark, et al. indicates that sorghum syrup production peaked at over 28 million gallons per year in the early 1880's, and peaked again to nearly 50 million gallons per year in 1920. Production continuously declined thereafter. Today, production is approximately 1 million gallons per year.
As described by R. L. Monk, et al., in "Improvement of Sorghum for Energy Production", supra, pp. 2-3, sorghum has many advantageous biological characteristics. It has a high photosynthetic rate and high drought tolerance. It is also capable of growing under high light and heat intensities. In addition, sorghum plants have a waxy surface which reduces internal moisture loss and facilitates drought resistance.
In recent years, a renewed interest in sweet sorghum has occurred. Primarily, research has been conducted on the use of sorghum as an energy source. This research has been directed to ethanol production from sorghum materials. Ethanol production from sugar-producing grasses including sweet sorghum is described in detail in E. S. Lipinsky, et al., "Sugar Crops as a Source of Fuels, Vol. I: Agricultural Research", Research Report for U.S. Dept. of Energy, Battelle Laboratories Columbus Division, July 31, 1978; E. S. Lipinsky, et al., "Sugar Crops as a Source of Fuels, Vol. II: Processing and Conversion Research", Research Report for U.S. Dept. of Energy, Battelle Laboratories Columbus Division, Aug. 31, 1978; and T. L. Dobbs, et al., "Alternative Crops for Ethanol Fuel Production: Agronomic, Processing, and Economic Considerations", Research Report 84-1 by Economics Department, South Dakota State University, April 1984.
Another way of using of sorghum as a fuel product involves combustion of solid sorghum waste materials. The removal of sugar-containing fluids from sorghum produces considerable quantities of ligno-cellulosic residue, otherwise known as "bagasse". According to Lipinsky, "Sugar Crops as a Source of Fuels--Vol. I: Agricultural Research", supra, p. 173, suggested uses for bagasse include pulp and paper manufacturing, and livestock feed. To produce energy, raw sorghum bagasse has been burned as a fuel source. For example, Jackson, et al., supra, page 80, suggests that sorghum waste products may be useful as combustible fuel. Research has also been conducted on the anaerobic digestion of sorghum to produce methane as described in E. A. Hiler, et al., in "Sorghums for Methane Production", Texas Agricultural Experiment Station, Texas A & M Univeristy, Annual Report, April 1983 -March 1984.
However, none of the above references describes inexpensive and effective methods to convert sorghum bagasse into usable fuel which is easily handled and sufficiently non-polluting. Moreover, none of the references discusses efficient and effective methods of storing and preserving unprocessed sorghum materials prior to conversion into fuel. The Hiler, et al. article discusses specific methods of storing harvested sorghum prior to digestion for producing methane. For example, pages 5-2, 5-3, 5-4 and 5-9 of the article describe various storage techniques, including silo storage of chopped material, baling and subsequent storage of field-dried material, rack storage of whole stalks, and chemical treatment of sorghum with preservatives including formic, propionic, and acrylic acid. All of these methods offer particular disadvantages. Silo storage requires expensive physical containment structures. It also requires the maintenance of strict anaerobic conditions to avoid bacterial spoilage. Baling requires a large quantity of physical labor. Chemical treatment is expensive and potentially hazardous. Finally, whole stalk storage is expensive, requires a substantial amount of storage space, and does not effectively inhibit microbial degradation.
To store bagasse, a common approach involved spraying water on bagasse piles, thereby maintaining high moisture levels in the material. This approach is called the Ritter method. Another method involved baling the bagasse in stacks which permit significant air flow through the material. However, both of these methods are expensive and result in heavy losses of biomass.
Because of its durability, low cost, and availability, sweet sorghum and its by-products represent a potentially valuable alternative energy source. However, a distinct need exists for methods to process these materials into efficient, non-polluting fuel. Also, a need exists for a method of storing the unprocessed sorghum materials which avoids undesirable oxidative degradation by microorganisms.